Hard thin films have become pervasive in advanced technology. Numerous industry groups in the US and the global economy will benefit from the availability of a reliable tool with high sensitivity for testing of wear of thin films. Specific industry groups that will benefit from improved methods and equipment for wear characterization include automotive, aircraft and other transportation industries, the electronics and metals manufacturing industry as well as the sector producing machinery for the chemical, food and other process industries. A relatively new area of coating applications that will also benefit is biomedical components. Improved methods for evaluating wear encompass economic benefits that relate to extending the lifetime of machinery and bio-systems, to making engines and devices more efficient, to conserving scarce material resources, to saving energy, and to improving safety.
A number of new superhard materials in the form of thin surface films (coatings) are being developed and produced. Dynamic mechanical characterization, and wear testing in particular, is necessary to assess the quality of such coatings both during development and fabrication of numerous products. In their recent review article on microtribology entitled "Recent Progress in Microtribology" and published in Wear, Vol. 200, pg. 296, (1996), Kaneko et al. describe the ultimate goal of microtribology namely, "to create practical zero-wear devices with very small mass and very light load". The authors concede that "we have very little of the knowledge needed for microtribology" and more data must be obtained.
Currently, standard ASTM (and VAMAS) tests are used for wear testing at the macroscopic scale. Nanoindentation measurements and Atomic Force Microscopy (AFM) are available for testing of mechanical properties of such films at the microscopic scale. Macroscopic testing relies on weight and/or height loss measurement. Both macroscopic and microscopic methods use profilometry of wear marks. The macroscopic methods of wear testing are not adequate, however, to provide meaningful test results for superhard thin films. Also, wear testing at the microscopic scale is both expensive and cumbersome.
The difficulty in film characterization using macroscopic methods arises from two primary factors: (1) the hard film material is often deposited on a less hard substrate, and (2) film thicknesses are commonly very small. As a result, such films are not well supported against a highly localized force such as that of a stylus or pin. Coating failure may often only be detected as a change in the friction force. The critical load, which is the load at which the film is scratched off, decreases with decreasing film thickness. Dynamic multipass tests which allow repeated passage of the same area have revealed that in practice, failure may occur at forces that are less than one-third of the critical load.
For these reasons, standard macroscopic approaches for tribological coating evaluation, such as the Pin-on-Disk test (ASTM G 99-95a, Annual Book of ASTM Standards, Vol. 03.02, 1997), are not appropriate since these lack the necessary sensitivity for the measurements which are required. The standard tests use detection methods that include height and weight loss evaluations, wear track profilometry, ball area measurements, and friction force measurements. Of these detection methods, only friction force measurement qualifies as a dynamic test. Taken alone, however, it is quite insufficient to describe coating quality. No direct relationship has been established between changes in friction force and wear quality. The height and weight measurements are also not well suited for extremely thin superhard films deposited on less hard substrate materials. Changes in height observed as indentations may be related to compression of the substrate and not to film wear and changes in weight may be very small and difficult to detect when extremely thin films are removed during testing. Methods for hardness testing of very thin films using displacement versus force diagrams have been developed but hardness is not always predictive for wear properties of superhard coatings. These films often contain multiple solid phases and the combination of phases of different hardness can significantly reduce the wear resistance.
Nanoindentation methods have been developed in the last fifteen years. See, for example, J. B. Pethica, "Microhardness Tests with Penetration Depths Less than Ion Implanted Layer Thickness", in Ion Implantation into Metals, Pergamon Press, Oxford, 1982, p. 147. In such methods, displacement of a stylus is continuously recorded as the applied load gradually increases and decreases. Further development of these methods has proceeded toward controlling ever decreasing loads and increasing the displacement sensitivity. Today, loads as low as 100 nN can be applied and penetration depths as small as 1 nm can be detected using either capacitance displacement sensors or optical fiber displacement sensors.
While this approach is very useful for hardness measurement, it does not provide information related to surface wear. Most nanoindenters provide for no motion of the specimen and thus yield no wear information. If motion of the specimen is allowed, measurement of the scratch produced by the stylus is performed by cross-sectional profiling. Continuous wear testing is also not feasible using this method.
The wear testing procedure using AFM involves the use of the AFM stylus for causing wear of the specimen. The wear, if any, is periodically tested by obtaining micrographs of the area influenced by the stylus. Continuous or quasi-continuous testing is not feasible. Another weakness of this method as applied to superhard materials is that the stylus is subject to wear during the test and thus subsequent AFM micrographs are subject to changes not only due to changes in the specimen (wear) but also due to changes in the measuring device.
Both AFM and nanoindenter instruments are well developed and are commercially available from a number of manufacturers. These instruments are quite expensive because of their highly technical and complex design. The complexity results from measuring very small mechanical displacements of a stylus, wherefore the instrument must be extremely well insulated from vibration. A broader industrial application of nanoindenter and AFM instruments is hindered not only by the expense and complexity of these instruments but also by the complexity of the information obtained. Neither of these techniques is mentioned in the context of wear testing in the recent text on tribology by K. Holmberg and A. Matthews entitled "Coatings Tribology", which was published by Elsevier, Amsterdam, 1994.
It has been established that high contact pressure, low speed, and a large number of passes are necessary to obtain statistically significant results in scratch-load testing. See, for example, S. Bennett and A. Matthews, "Multifunction Scratch Tester", Surface and Coatings Technology, Vol. 75, p. 869 (1995). These requirements are very difficult to implement. In the case of AFM and nanoindenters, the achievement of statistically significant results would require making many scratches and going back to analyze maps of each of them. The underlying problem is that in these methods one obtains too much information about the detailed shapes of scratches. This information is excessive in the sense that it is not necessary for establishing the presence of wear. However, it may be useful information for other research purposes.
Although there is interest in obtaining information about the wear in a continuous fashion, neither the macroscopic nor the microscopic methods are adequate to provide such data. The ASTM G 99-95a Standard expressly warns against using continuous wear depth data collection using position-sensing gages because of the errors associated with such measurement. This warning applies both to the use of machine vision devices and interferometers to obtain continuous wear test data.
Light scattering measurement has not been previously used in the area of wear testing. The light scattering technique for surface roughness measurement was developed more than 20 years ago. The variation of the measurement of off-specular light scattering with the collection of the off-specular light using a hemispherical mirror is called total integrated scattering (TIS) technique and henceforth will be referred to with this acronym. The measurement of surface features using TIS on small surface areas has been demonstrated to allow surface roughness measurements with a sensitivity of 1 nm rms or better, as set forth in U.S. Pat. No. 4,954,722 and the article "Scanning Scattering Microscope for Surface Microtopography and Defect Imaging", Vol. 14, Journal of Vacuum Science and Technology B, pg. 2417 (1996), by J. Lorincik et al.
None of the prior art techniques discussed above are well suited for easy and reliable wear testing of films and particularly very hard thin films. The disadvantages of the prior art are overcome by the present invention. Improved methods and apparatus are hereinafter disclosed which are relatively simple and may be easily, economically, and reliably operated to test the wear of various materials.